How Does the Block Reward Subsidy Affect the Economic Incentive for Selfish Mining?

The block reward subsidy (the newly minted coins) is the largest component of a miner's revenue, especially in the early stages of a cryptocurrency's life. A larger subsidy increases the economic incentive for all mining, including selfish mining, as the potential gain from disproportionately acquiring block rewards is higher.

As the subsidy decreases (e.g. through halving), the focus shifts more to transaction fees.